obscache: extract the actual data update in a dedicated function This will help extract a data agnostic class.

obscache: move an assert to a lower level

obscache: use 'nullid' as the hash of an empty obsstore This align this result with what we use for 'emptykey'

obcache: move empty on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obcache: move _checkkey on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obcache: move updateneeded on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obshistory: only display each revision once in debugobshistory When using a revision range, the same precursor could have been displayed more than once. Use a cache to display each revision only once. It's a fix until we found the nicest way to display obs history.